Recycling of cured aminoplast resins

ABSTRACT

Cured aminoplast resins are recycled by a process in which said resins, which may contain fillers, are treated with an aqueous sulfite, hydrogen sulfite or disulfite solution and the resulting low molecular weight degradation products are used in chemical processes. The resulting low molecular weight degradation products of cured aminoplast resins are used for coating moldings, for tanning leather and skins and for the production of moldings which contain cured aminoplast resins or consist thereof.

The present invention relates to a process for recycling curedaminoplast resins, in particular for recycling moldings which containcured aminoplast resins or consist thereof.

Various processes can be used for recycling polymers. A process which isnow frequently used comprises mechanical comminution of the polymer,which as far as possible is of one type, and subsequent thermoplasticforming. However, this is feasible only to a limited extent in the caseof cured aminoplast resins. Although the cured aminoplast resin can bemechanically comminuted, thermal forming is impossible. It is possibleonly to bind the milled resin powder by means of suitable additives togive new moldings.

The aminoplast resins include urea resins and triazine resins. Suchresins are increasingly being processed to moldings, such as fibers andfoams (cf. DE-A-23 64 091, DE-A-27 54 525, DE-A-29 15 457, DE-A-41 23050 and EP-A-408 947). However, this also gives rise to the question ofrecycling of these products.

It is an object of the present invention to provide a process forrecycling cured aminoplast resins which permits the chemical recyclingof moldings which contain aminoplast resins or consist thereof.

We have found that this object is achieved by a process for recyclingcured aminoplast resins, in which cured aminoplast resins which maycontain fillers are treated with an aqueous sulfite, hydrogen sulfite ordisulfite solution and the resulting low molecular weight degradationproducts are used in chemical processes.

The present invention furthermore relates to low molecular weightdegradation products of cured aminoplast resins, which are obtainable bytreating cured aminoplast resins which may contain fillers with anaqueous sulfite, hydrogen sulfite or disulfite solution.

The present invention also relates to the use of these low molecularweight degradation products for coating moldings, preferably particleboards, and for tanning hides and skins and for the production ofmoldings which contain aminoplast resins or consist thereof.

German Patent Application P 42 42 046.6 discloses a process for thechemical degradation of cured aminoplast resins, in which said resins,which may contain fillers, are reacted with a primary and/or secondaryamine at from 40° to 250° C.

DD 155 779 discloses a process for removing cured aminoplasts fromproduction plants, in which the cured aminoplasts are dissolved withaqueous, 1-70% strength by weight ammonium, alkali metal or alkalineearth metal sulfite solutions or ammonium, alkali metal or alkalineearth metal hydrogen sulfite solutions or mixtures of these solutions atfrom 20° to 100° C., and the ratio of aminoplast to be dissolved tosolvent is maintained at from 1:1 to 1:10 in the dissolution process.

According to the invention, cured aminoplast resins are, for example,condensates of urea, melamine, benzoguanamine, acetoguanamine,dicyandiamide, guanidine or thiourea with aldehydes, in particular withformaldehyde.

Preferred aminoplast resins are urea resins and/or triazine resins. Inthe case of the triazine resins, the triazine component is chosen, forexample, from the group consisting of benzoguanamine, acetoguanamine andmelamine.

Melamine resins are preferably used. Particularly preferred melamineresins here are the condensates of melamine or melamine derivatives andformaldehyde, as described in, for example, EP-A-221 330 and EP-A-408947.

The aminoplast resin used in the novel process is generally obtained bymechanical comminution of moldings of aminoplasts, for example fibersand foams. The finely divided aminoplast resin obtained can then be usedin the novel process without further pretreatment.

The fillers present in the moldings consisting of aminoplasts generallydo not present problems here.

Here, fillers are in general fibrous or pulverulent inorganicreinforcing agents or fillers, such as glass fibers, metal powder, metalsalts or silicates, e.g. kaolin, talc, barite, quartz or chalk, andpigments, dyes and flameproofing agents may also be present.

According to the invention, the cured aminoplast resin is reacted ingeneral with from 1 to 80, in particular from 20 to 40%, strengthaqueous alkali metal, alkaline earth metal or ammonium sulfite solutionor alkali metal, alkaline earth metal or ammonium hydrogen sulfitesolution at from 20° to 150° C. The weight ratio of solvent toaminoplast resin depends on the corresponding resin and on theconcentration of the solution. It is as a rule from 1:1 to 1:30. The pHis generally from 4 to 7. The reaction time is dependent on thetemperature and is in general from 3 to 60 hours. At above 100° C.,superatmospheric pressure is advantageously employed.

The reaction time is furthermore dependent on the surface of the resinused.

In general, the reaction takes place in such a way that a clear solutionis formed after from about 1 to 8 hours, depending on the surface of themilled resin. When melamine/formaldehyde resins are used, precipitationof monomeric melamine derivatives occurs as the reaction progresses,even at about 100° C. After the end of the reaction and cooling to roomtemperature, these products can be removed in a simple manner, forexample by filtration. The mother liquor can be used again for furtherreactions.

In the case of melamine/formaldehyde resins, the white powder removed byfiltration is a defined mixture of methylolemelamines and thecorresponding methylolmelamines modified with sulfite groups. The numberof methylol groups depends on the melamine/formaldehyde ratio of theresin used. The recovery of the melamine building block from the resinis in general from 70 to 95%.

The use of urea resins generally results within a short time in clearsolutions, from which, as a rule, no monomeric products areprecipitated. As the reaction progresses, a defined mixture ofmethylolureas and methylolureas modified with sulfite groups forms.

These solutions can advantageously be used directly in the case ofcertain reactions. Alternatively, particularly the methylolureas can beprecipitated from sufficiently concentrated solutions after the hotsolutions have been cooled and can be filtered off. In general, about50% of the urea present in the crosslinked urea resin can be recoveredin this manner. This result is further improved by virtue of the factthat the filtrate solution can be used directly for further reactions.

The novel process can also be carried out in the presence of awater-soluble organic solvent (for example an alcohol).

Cured aminoplast resins, in particular melamine resins, can beconverted, according to the invention, into correspondinglyfunctionalized monomeric derivatives which are simple to isolate and canbe used for various chemical processes.

It is thus possible to recycle moldings consisting of aminoplast resinsin a simple and economical manner.

The novel functionalized monomeric derivatives (low molecular weightdegradation products) can be used in chemical processes in the form ofthe solution present after the novel treatment or in the form of theisolated solid products. Depending on the working up of the solution,the product fractions obtained contain methylol and sulfite substituentsin different amounts.

According to European Patent Application No. 92 108 389.5, themethylol-rich product fractions can be incorporated into condensateswhich can be used as tanning agents for tanning pelts and skins and forretanning leather and skins.

Pelts and skins are understood here as meaning the untanned hide freedfrom epidermal and subcutaneous connective tissues.

The methylol-rich product fractions are generally used in an amount offrom 1 to 5% by weight, based on the condensate.

Furthermore, the novel low molecular weight degradation products canserve as condensation components, in general in an amount of from 1 to5% by weight, based on the resin, in adhesive resins and impregnatingresins, as used in particular for gluing lignocellulose-containingmoldings.

A further example is the use of the novel low molecular weightdegradation products as modifiers in melamine/formaldehyde resins, asdescribed in DE-A-29 15 457, DE-A-23 64 091, EP-A-221 330 and DE-A-41 23050. These resins can subsequently be spun, for example, into textilefibers.

In the Examples which follow, parts and percentages are by weight.

DEGRADATION OF AMINOPLAST RESINS (EXAMPLES 1 TO 4) Example 1

20 g of a milled, cured melamine/formaldehyde resin having a molar ratioof melamine to formaldehyde of 1:3 (M:F=1:3) were stirred in 200 g of40% strength sodium disulfite solution at 100° C. After 5 hours, a clearsolution formed, which became very cloudy after a further 20 hours. 48hours after the beginning of the reaction, the solution was cooled toroom temperature and the white precipitate was filtered off, washed with10 g of water and dried.

Yield: 12.5 g

According to HPLC analysis (evaluation of the areas), the precipitateconsisted of:

    ______________________________________                                        Dimethylolmelaminesulfonic acid                                                                      28 mol %                                               Monomethylolmelaminesulfonic acid                                                                    53 mol %                                               Dimethylolmelamine      9 mol %                                               Monomethylolmelamine   10 mol %                                               ______________________________________                                    

Example 2

3.42 kg of a milled, cured melamine/formaldehyde resin (M:F=1:3) werestirred in 49 kg of 20% strength sodium disulfite solution at 100° C.After 6 hours, a clear solution formed, which became very cloudy after afurther 12 hours. 24 hours after the beginning of the reaction, thesolution was cooled to room temperature and a white precipitate wasfiltered off, washed with 3 1 of water and dried.

Yield: 3.04 g

According to HPLC analysis (evaluation of the areas), the precipitateconsisted of:

    ______________________________________                                        Dimethylolmelaminesulfonic acid                                                                      20 mol %                                               Monomethylolmelaminesulfonic acid                                                                    50 mol %                                               Dimethylolmelamine     15 mol %                                               Monomethylolmelamine   15 mol %                                               ______________________________________                                    

Example 3

5 g of a milled, cured melamine/formaldehyde resin (M:F=1:2) werestirred in 200 g of 20% strength sodium disulfite solution at 100° C.After 8 hours, an almost clear solution formed, which became very cloudyafter a further 20 hours. 48 hours after the beginning of the reaction,the solution was cooled to room temperature and a white precipitate wasfiltered off, washed with 10 g of water and dried.

Yield: 2.7 g

According to HPLC analysis (evaluation of the areas), the precipitateconsisted of:

    ______________________________________                                        Dimethylolmelaminesulfonic acid                                                                       7 mol %                                               Monomethylolmelaminesulfonic acid                                                                    44 mol %                                               Dimethylolmelamine     10 mol %                                               Monomethylolmelamine   35 mol %                                               Melamine                4 mol %                                               ______________________________________                                    

Example 4

10 g of a milled, cured melamine/formaldehyde resin (U:F=1:1.55) werestirred in 120 g of 20% strength sodium disulfite solution at 100° C.After 3 hours, a clear solution formed. It was stirred for 48 hours at100° C. and then cooled to room temperature.

The evaluation by quantitative ¹³ C-NMR gave the following compositionof the urea derivatives:

    ______________________________________                                        Methylolurea       30%                                                        Sulfomethylated urea                                                                             70%                                                        ______________________________________                                    

USE OF THE LOW MOLECULAR WEIGHT DEGRADATION PRODUCTS (EXAMPLES 5 TO 7)Example 5 Condensates for Tanning Leather and Skins

141 g (1.5 mol) of phenol were stirred with 168 g (1.65 mol) ofconcentrated sulfuric acid for 3 hours at 105° C. After the addition of69 g of water, 81 g (1.35 mol) of urea and 8.13 g (1.86% by weight) ofsulfite-modified methylolmelamines (product from Example 3) were added.In addition, 278 g (2.78 mol) of 30% strength aqueous formaldehyde weremetered in over 100 minutes at from 65° to 70° C. Stirring was carriedout for 15 minutes, after which the pH was brought to 2.0 at from 65 to70° C. with 138 g (1.73 mol) of 50% strength NaOH. The solution-wasstirred at 90° C. for 2 hours and then brought to a pH of 7.0 with 22.5g (0.28 mol) of 50% strength aqueous NaOH. The pH was brought to 3.6 byadding 69 g of a mixture of 30 parts of succinic acid, 42 parts ofglutaric acid and 28 parts of adipic acid. The solids content wasbrought to 40% with 340 g of water.

Viscosity (20° C.): 28 mPa.s pH: 3.6

Example 6 Condensation Resin for Coating Particle Boards

219 g of melamine, 6.2 g of sodium disulfite, 11.5 g of a methylolcompound (product from Example 3), 208 g of 40% strength aqueousformaldehyde, 18.9 g of butane-1,4-diol, 4.8 g of ethylene glycol, 126 gof water and 1.3 g of sulfanilamide were mixed. The mixture was broughtto pH 8.45 with NaOH, and 0.9 ml of diethylethanolamine was added. Themixture was heated to 105° C. until the solution became clear, afterwhich the temperature was reduced to 96° C. Condensation was carried outuntil a turbidity point TRP (1:5) of 47° C. was reached. Here, turbiditypoint TRP (1:5) means the temperature at which precipitation occurs in amixture of resin solution and water in a weight ratio of 1:5.

pH: 10.5 d=1.2359 g/cm³ Viscosity (20° C.)=53 mPa.s Solids content=55%

Example 7 Melamine/Formaldehyde Condensation Resin with the Addition ofMethylolated Compound

170.1 g (0.9 mol) of melamine and 61.7 g of an 80% strength aqueoussolution of N,N',N"-tris-(5-hydroxy-3-oxapentyl)-melamine (0.1 mol) weremixed with 45.8 g of paraformaldehyde, 3.72 g of bisphenol A, 1.5 ml ofdiethylaminoethanol and 110.7 g of 40% strength aqueous formaldehydesolution and 1.6 g of a methylol compound. The reaction mixture wasrefluxed until it had a viscosity of 630 Pa.s (98° C.).

The spinning of the condensation resin to fibers was carried outaccording to DE-A-41 23 050.

We claim:
 1. A process for recycling cured aminoplast resins, whereincured aminoplast resins, which may contain fillers, are treated with anaqueous sulfite, hydrogen sulfite or disulfite solution resulting in theproduction of low molecular weight degradation products comprisingmethylolureas, methylolamino substituted 1,3,5-triazines andmethylolamines modified with sulfite groups.
 2. A process as claimed inclaim 1, wherein the cured aminoplast resins used are urea resins ortriazine resins.
 3. A process as claimed in claim 2, wherein thetriazine resins are melamine resins.
 4. A low molecular weightdegradation product of cured aminoplast resins, obtained by treatingcured aminoplast resins, which may contain fillers, with an aqueoussulfite, hydrogen sulfite or disulfite solution.